Various electro-optical systems have been developed for reading optical indicia, such as bar codes. A bar code is a coded pattern of graphical indicia comprised of a series of bars and spaces of varying widths, the bars and spaces having differing light reflecting characteristics. Some of the more popular bar code symbologies include: Uniform Product Code (UPC), typically used in retail stores sales; Data Matrix, typically used for labeling small electronic products; Code 39, primarily used in inventory tracking; and Postnet, which is used for encoding zip codes for U.S. mail. Bar codes may be one dimensional (ID), i.e., a single row of graphical indicia such as a UPC bar code or two dimensional (2D), i.e., multiple rows of graphical indicia comprising a single bar code, such as Data Matrix which comprising multiple rows and columns of black and white square modules arranged in a square or rectangular pattern.
Systems that read bar codes (bar code readers) electro-optically transform the graphic indicia into electrical signals, which are decoded into alphanumerical characters that are intended to be descriptive of the article or some characteristic thereof. The characters are then typically represented in digital form and utilized as an input to a data processing system for various end-user applications such as point-of-sale processing, inventory control and the like.
Bar code readers that read and decode bar codes employing imaging systems are typically referred to as imaging-based bar code readers or bar code scanners. Imaging systems include charge coupled device (CCD) arrays, complementary metal oxide semiconductor (CMOS) arrays, or other imaging sensor arrays having a plurality of photosensitive elements (photosensors) defining image pixels. An illumination apparatus or system comprising light emitting diodes (LEDs) or other light source directs illumination toward a target object, e.g., a target bar code. Light reflected from the target bar code is focused through a system of one or more lens of the imaging system onto the sensor array. Thus, the target bar code within a field of view (FV) of the imaging lens system is projected on the sensor array.
Periodically, the pixels of the sensor array are sequentially read out generating an analog signal representative of a captured image frame. The analog signal is amplified by a gain factor and the amplified analog signal is digitized by an analog-to-digital converter. Decoding circuitry of the imaging system processes the digitized signals representative of the captured image frame and attempts to decode the imaged bar code.
Imaging-based bar code readers which are portable, as opposed to a fixed mounted or stationary reader, typically have housings that designed to be held by a user in his or her hand and pointed at a target bar code by the user to image and decode the target bar code. To facilitate the user appropriately pointing or aiming the bar code reader at the target bar code, readers are often provided with an aiming pattern assembly which generates a visible aiming pattern. The generated aiming pattern may be a visible dot projected approximately in the middle of the field of view FV of the imaging system. Alternately, the generated aiming pattern may be a visible crosshair aiming pattern with the vertex of the crosshair pattern projected approximately in the middle of the field of view FV of the imaging system. Yet another alternative would be for the generated aiming pattern to comprise a rectangular or circular pattern of visible illumination whose center is projected approximately in the middle of the field of view FV of the imaging assembly and whose outer periphery approximates a periphery of the field of view FV, referred to herein as a block-type aiming pattern. A block-type aiming pattern indicates to the user the extent of the field of view FV so that the user can appropriately aim the reader housing such that the target bar code is within the bounds of the imaging assembly field of view FV.
The aiming pattern assembly is typically offset from the imaging system that is perpendicular to an optical axis of the imaging system. Because the aiming pattern assembly is not coaxially aligned with the imaging system in the reader housing, this results in parallax between the imaging system field of view FV and the aiming pattern. While there may be one target distance at which a center of the aiming pattern and a center of the field of view FV of the imaging assembly are congruent, because of parallax, at other target distances, the center of the aiming pattern will be shifted with respect to the center of the field of view FV. Additionally, for a block-type aiming pattern, because of parallax, the peripheral bounds of the aiming pattern will not correspond with the peripheral bounds of the field of view FV.
As reader imaging systems are improved to have greater and greater working ranges (WR), that is, the ability to read target bar codes over distances from a few centimeters from the front of the imaging system to distances of several meters from the imaging system, the impact of the parallax error on the lack of congruency between the aiming pattern and the field of view FV is exacerbated. For example, one way to increase the effective working range WR of an imaging system is move from a fixed focus lens system to a variable focus lens system. In a fixed focus system, the field of view FV and the working range WR of the imaging system is fixed. The working range WR of an imaging system is a distance range in front of or forward of the imaging lens assembly within which a target object of interest, such as a target bar code, may be successfully imaged and decoded by the imaging system decoding circuitry.
The working range WR and field of view FV require a user to move the bar code reader relative to the target bar code such that the target bar code is within the field of view FV and within the working range WR of the imaging system for successful decoding of the imaged target bar code. At the near and far limits of the working range WR, there is a problem with blurriness, that is, poor resolution of the imaged target bar code. A variable focus lens system addresses the blurriness problem at the limits of the working range WR by providing for readjustment/refocusing of the lens system. This readjustment/refocusing of the lens system facilitates obtaining an in-focus image of a target bar code focused onto the sensor array, thus, mitigating blurriness at the limits of the working range WR. A disclosure of a variable focus lens assemblies is presented in U.S. patent application Ser. No. 11/756,052, filed May 31, 2007 and entitled “Variable Focus Imaging Lens Assembly For An Imaging-Based Bar Code Reader”. The aforesaid '052 application is assigned to the assignee of the present invention and is incorporated herein in its entirety by reference.
In addition to parallax, a variable focus imaging lens assembly adds additional complications to problem of the aiming pattern being congruent with the imaging assembly field of view FV. For example, in a variable focus imaging lens assembly that employs a zoom lens, the angle of the field of view FV varies between a narrow angle field of view and a wide angle field of view depending on the position zoom lens. Accordingly, a block-type aiming pattern that is appropriate for a narrow angle field of view FV would be inappropriate for a wide angle field of view FV. That is, the periphery of the block-type aiming pattern would not accurately represent both the bounds of the narrow angle field of view and the wide angle field of view.
What is needed is an aiming pattern assembly for an imaging-base bar code reader that provides for multiple aiming patterns whose center lines more accurately correspond to a center line of the field the field of view FV over the working range WR of the imaging system. What is also needed is an aiming pattern assembly for an imaging-base bar code reader that provides for multiple aiming patterns that more accurately correspond to the field of view FV over the working range WR of the imaging system. What is also needed is an aiming pattern assembly for an imaging-base bar code reader that provides for multiple aiming patterns that more accurately correspond to wide and narrow angle fields of view FV over the working range WR of the imaging system when a zoom lens imaging system is utilized.